This invention relates to a calibrating apparatus and method for detecting a light sensitive column within a transparent body and particularly to devices employing a fluid column of a temperature sensitive medium.
Tube-type thermometers of a conventional design employ an elongated transparent tube having a small diameter bore within which an expanding liquid is confined. The lower end of the tube is formed as a bulb forming an enlarged chamber connected to the bore. Oil is advantageously employed as an effective and low cost expandable liquid. Oil does, however, have a non-linear expansion characteristic compared to, for example, mercury and certain gases which might also be employed. In commercial production, the bulb volume and average bore diameter of each tube may vary and each tube will have its own unique characteristic dependent on the particular bulb volume and bore diameter.
Historically, each tube is "scaled" to determine the actual length of travel of the oil column for a given temperature change. The tubes are then sorted and stored by whole scale numbers. The number of millimeters of the column travel between the 32.degree. F. point and the 92.degree. F. temperature point of a given tube and can be used as the scale number of that tube. Historically, in the manufacture of thermometers, the tubes are formed with open tops and filled with the oil. The tube is then "heated out" to a higher than readout temperature to expand the oil to overflow.
Typically, "heat-out" temperature is either 130.degree. F. or 150.degree. F. The volume of fluid is thus fixed, with the particular heat-out temperature employed merely to vary the location of the actual tube scale on the tube. The tops of the heated out tubes are then sealed. They are then manually scaled and sorted by whole scale numbers for final application of a scale to the thermometer tubes. Finally in an additional manual process, each tube is "marked" at a convenient temperature point such as 70.degree. F. so that it can be attached accurately to its appropriate scale.
Although relatively low cost, reasonably accurate and reliable thermometers are so produced, the labor intensive scaling and working process adds significantly to production costs and involves the usual problems of unavoidable human error.